From what I have read here, the underlying theory is that someone younger doesn't have kids/spouse/family/etc. So they are more willing to work long hours (I guess without pay since salaried?). Also the theory is that someone older means they would want more money for their experience.

Note that I don't actually know, this seems to be the running theory as I understand it.

Why do they have to get something out of it? People practice discrimination for all sorts of illogical reasons, rarely profiting from it.

My father was a victim of ageism before there were any laws against it. A new young CEO let everyone go over the age of 55. It was a Fortune 500 company that had been around for over a century and this tanked the entire company, they never recovered.

This company let ten percent of their staff go each year. These people over 55 were company lifers because they knew how to get stuff done. They wiped out a large percentage of their elite employees all at once. Replacing them with new hires right out of college was too much of a toxic shock to the company all at once.

There's also an epidemic of imposter syndrome blooming within these "competitive" tech companies.

https://www.quora.com/Do-engineers-at-companies-like-Google-...

I put "competitive" in quotes, because you can't really have actual, honest competition if you exclude (or include fewer) African Americans, women, and people over a certain age who are "not a culture fit", as it says in the article. In that linked Quora post, it mentions a serious problem with this syndrome at Google in particular, and an internal list dedicated to it. When one is relatively young or inexperienced and one has a kickass job plus imposter syndrome, one might find it unsettling to be the boss of someone older/more experienced or perhaps more accomplished than one... (see Dan Lyons comedic output). ONe way around this is to avoid hiring anyone that distant from your own experience level.